The present invention relates to a gas injection device and to a method for injecting a gas into a processing chamber. The present invention relates in particular to a method for injecting a gas into a processing chamber of a thin film reactor.
Thin film deposition techniques, such as Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD), are techniques for depositing thin film layers upon a substrate, such as a semiconductor substrate. One particular CVD process subclass, called Atomic Layer Deposition ALD (also known as Atomic Layer Epitaxy ALE or Atomic Layer Chemical Vapor Deposition ALCVD), is used for semiconductor and thin film magnetic head manufacturing, and is being considered for the manufacturing of various new devices such as organic light emitting displays (OLED's) and photovoltaic elements. FIG. 1A shows a conventional thin film deposition system TFS1 comprising at least one injector 1 with a flow-shaping section 2, a processing chamber 4 wherein a substrate 5 may be placed, and an exhaust device 6. A gas tube 7 links the injector 1 to at least one source gas 8.
During a deposition process, a carrier gas comprising reactants is generally introduced into the injector 1 during a certain period of time, thereby forming a “gas wave” or “gas pulse”. As described in U.S. Pat. No. 7,163,587, the flow-shaping section 2 of the injector may have a triangular shape with first and second sidewalls diverging according to a constant divergence angle relative to a propagation axis XX′ of the gas wave inside the injector. The flow-shaping section 2 laterally expands the gas wave as it travels from a point O at the inlet of the flow-shaping section 2 until it reaches an outlet of the injector that opens onto the processing chamber 4. The gas is then expelled into the processing chamber 4, as shown by arrows in FIG. 1A, wherein reactants in the gas may react with the substrate surface 5 and/or with previously-deposited molecules. The processing chamber 4 may then be purged by injecting an inert gas that clears any excess reactants and products from the system, which are evacuated by means of the exhaust device 6. The pulsing/purging steps may then be repeated with a second gas from another gas source. Thin layers, for example between 0.1 and 3 Å, may be formed upon the substrate 5. This cycle is repeated as many times as necessary to obtain the desired thin film thickness.
Due to its layer-by-layer implementation, Atomic Layer Deposition allows for very high structural quality and thickness control of the thin film layers, as well as good step coverage over any features that may be present on the substrate. However, due to the required pulsing and purging steps, this process may take anywhere from several minutes to several hours, depending upon many factors such as the desired thin film thickness, reactants used, rapidity of the cycling, etc., resulting in a relatively low throughput. Recent research and development has focused on decreasing the deposition time of thin films in order to make this technique more attractive for large-scale production.
One common way to decrease the cycle time is to increase the gas flow rate. However, due to the Poiseuille effect, the triangular shape of the flow-shaping section causes the gas in the center of the injector to arrive at the outlet before the gas near the sidewalls. Thus, when such an injector is used in an application where a time-sequenced composition profile is created at the entrance, this peaked velocity distribution will result in a non-uniform gas composition distribution. Such a non-uniform gas distribution slows down the process by increasing the amount of time for a gas wave to travel through the processing chamber, as will be explained in relation with FIGS. 1B, 1C.
FIG. 1B shows the profile C01 of the gas velocity V at the outlet of the injector 1 of FIG. 1A. The gas velocity is measured along an axis YY′ perpendicular to the gas propagation axis XX′ and is expressed in meters per second. It can be seen that the gas velocity profile C01 has a peak value at a point O′ at the center of the outlet of the injector, and quickly decreases when going away from point O′, to reach 0 at the vicinity of the sidewalls of the injector. FIG. 1C shows two profiles P01, P02 of gas concentration GC along the propagation axis XX′ through the processing chamber 4, from point O′ at the outlet of the injector, at two different times after the injection of the gas wave. The gas concentration is expressed in percentage of the reactant present in the gas per unit of volume. It can be seen that, due to the diffusion of the reactant in the carrier gas, the length of the gas wave or gas pulse increases as it travels through the processing chamber.
In view of FIGS. 1B, 1C, it can be understood that a relation exists between the length of the gas wave P01 and the gas velocity profile C01, such that the more non-uniform the gas velocity profile along YY′ at the outlet of the injector, the longer the time to expel the gas from the injector, the longer the profile of the gas wave along XX′ that travels through the processing chamber, and the longer the time required between the injection of two successive gas pulses, since the slowest part of the gas wave must exit the chamber before the next wave is injected.
A non-uniform gas distribution may also cause deposition anomalies and uneven thin films. Therefore, it may be desirable to have as uniform a distribution of the gas across the surface of the substrate as possible. The gas wave should be optimized for the thin film deposition system and is highly dependent upon the physical properties of the injectors, dimensions of the processing chamber, substrate to be deposited upon, etc. As thin film deposition systems are typically used for electronic applications, they are therefore optimized for the standardized substrate dimensions often used in this industry, such as 150, 200, and 300 mm diameters. In order to apply these techniques to larger substrate sizes for other application domains, the dimensions of the injectors, gas inlets, processing chamber, exhaust device, etc. must be modified accordingly, complicating and increasing the cost of thin film deposition systems for large (500 mm or more) substrate sizes with fast cycling times. It is neither practical nor economical to develop a system for each possible processing combination, and current systems are not well-adapted for larger substrates, limiting their utility for other domains of application.
Furthermore, recent interest has arisen for the application of thin film deposition methods to other industries such as glass coating, display, and photovoltaics. These applications use much larger substrates, such as 1200 by 600 mm glass plates or a continuous roll of flexible material, requiring an increase in the amount of gas traveling through the injector, and a widening of the outlet of the injector. However, increasing the gas flow rate and widening the outlet of the injector cause turbulence and recirculation of the gas in the injector, resulting in an even more non-uniform distribution of the gas across the substrate surface and inefficient purging of precursors from the system.